Blower door test reveals extent of building air leakage.

		More About Air Leakage
		As the difference between indoor and outdoor air pressure in a building grows, air will leak out. The top areas and lowest floors of a building are areas with the biggest air pressure differences. Because warm and cold air rise and fall in a building, leaks in these areas are typically greater than elsewhere. The warm inside air will rise and leak out any unsealed areas near the top of the building. Cold outside air will seep in through breaches near the bottom of the building. If properly ventilated, relatively airtight buildings are more cost-efficient and provide better occupant comfort. Occupants experience fewer drafts, and it costs far less to maintain comfortable air temperatures and healthy air circulation. Proper airflow also helps preserve the quality of the building’s materials. As moist, heated air leaks into cooler insulated walls, ceilings and roofs, the warm vapor condenses on the cool surfaces creating an ideal environment for the growth of mold, mildew, and other unhealthy condensation conditions. These conditions can have an adverse effect on anyone with allergies and can be a contributor to “Sick Building Syndrome.”

By placing the building under a known pressure, a Blower Door test allows the technician to measure how much building airflow is required to maintain a certain pressure differential between indoors and outdoors. The number of air changes per hour (the complete replacement of the indoor air volume of the building in one hour) at a standard pressure differential (interior pressure compared to exterior pressure, measured in Pascals) reveals the building envelope's air leakage profile.

Blower Door measurements can also be used to estimate the natural infiltration rate of buildings. While the Blower Door doesn't measure infiltration rates directly, test results can be used along with mathematical models to estimate annual average and design infiltration rates for the purposes of estimating the efficiency of the building envelope, evaluating indoor air quality, estimate the need for mechanical ventilation, and to help with proper sizing of heating and cooling equipment.

A Blower Door consists of powerful variable-speed fan units mounted in an adjustable panel that temporarily fits in a doorway. Pressure gauges connected to the fan measure the rate of airflow required to maintain the building at a certain pressure. This controlled airflow enables the technician to both find and measure air leakage in the building envelope.

A basic Blower Door system includes three components: calibrated fan units, a door-panel system, and a device to measure fan flow and building pressure. The Blower Door fan is temporarily sealed into an exterior doorway using the door-panel system. The fan is used to blow air into or out of the building, which creates a small pressure differential between inside and outside. This pressure imbalance forces air through all holes and penetrations in the building enclosure.

By measuring the airflow through the fan and its effect on the air pressure in the building, the Blower Door test measures the air tightness of the entire building envelope. The tighter the building (e.g. fewer holes), the less air the Blower Door must move to maintain a given pressure. Blower Door tests are typically performed at a pressure difference of 50 Pa (0.2 inches of water column), although in some instances, larger and/or more leaky buildings may not allow the unit to achieve the ideal pressurization levels.

Air tightness measurements are presented as number of air changes per hour at 50 Pa of pressure difference (ACH50). A typical air movement rate for a newly constructed building is approximately .25 air changes per hour. The test assigned an estimated average annual infiltration rate for this building at .43 air changes per hour, making this building significantly more leaky than a newly constructed building.